Differences between Machine Capability and Product Capability studies

M

Mumus

#1
Hi All,

I'm a bit confused....
One of our customers are asking us to do a Machine Capability study !!!
And I asked if they reffer to Cpk capability study (on product) and said NO.

Can somebody explain the differences between Machine capability and Product capability studies and how to perform (machine one) ?

Thanks a lot...:notme:

M.
 
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ScottK

Not out of the crisis
Leader
Super Moderator
#2
Re: Machine Capability ?

I would guess that they want to know the output of the machines in question.
i.e. cycle times,
production numbers per shift/day/week, etc.
maybe changeover time
perhaps standard scrap rate, especially if the customer is providing raw materials (in contract packaging we always had to account for every drop of product including that left in the lines after a production run)
 
M

Mumus

#3
Re: Machine Capability ?

Discordian said:
I would guess that they want to know the output of the machines in question.
i.e. cycle times,
production numbers per shift/day/week, etc.
maybe changeover time
perhaps standard scrap rate, especially if the customer is providing raw materials (in contract packaging we always had to account for every drop of product including that left in the lines after a production run)
They also mentioned that they would expect nothing less than 1.33...which is for Cpk as far as I know.
I'm confused :(
 
#5
Mumus said:
Can somebody explain the differences between Machine capability and Product capability studies and how to perform (machine one)
They are probably asking for Cmk, which is calculated in the same way as Cpk. The difference is that the data comes from a special study aiming to exclude variation caused by anything but the machine (You use the same operator, material from the same batch, unchanged setup, and so on).

Have a look at Machine Capability vs Process Capability

/Claes
 
P

pavankumar

#7
I am presently doing masters in industrial engg. and presently got a doubt with my project. I am taking into consideration of two machines one of which is a traditional lathe and other being a cnc m/c. I would like to develop a sampling plan. Is there a realtion between sampling size and machine capability and also does machine capablity infuence the sampling size. if so how. . also based on defects how is machine capablity calculated???
 

Jim Wynne

Leader
Admin
#8
I am presently doing masters in industrial engg. and presently got a doubt with my project. I am taking into consideration of two machines one of which is a traditional lathe and other being a cnc m/c. I would like to develop a sampling plan. Is there a realtion between sampling size and machine capability and also does machine capablity infuence the sampling size. if so how. .
Welcome to the Cove :bigwave:

If you're trying to empirically determine whether to use a simple lathe or a CNC machine, the initial sampling must be the same for both. Once you know something about the variation induced by each process, you can make a decision on which machine to use (which, by the way, involves other factors, such as costs of operation). In other words, you have to know something about machine capability before you arrive at a production sampling plan. If you run the same job on two machines with different inherent capabilities, sampling might have to be more frequent on the less capable machine. How much more frequent is function of your knowledge of the variability, and there is no single formula for deciding when and how to sample.

also based on defects how is machine capablity calculated???
Have a look further up the thread for some links, and at the bottom of this page for links to similar threads.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#9
I am presently doing masters in industrial eng. and presently got a doubt with my project. I am taking into consideration of two machines one of which is a traditional lathe and other being a cnc m/c. I would like to develop a sampling plan. Is there a relation between sampling size and machine capability and also does machine capability influence the sampling size. if so how. . also based on defects how is machine capability calculated???
First of all, it is important to determine your distribution. In precision machining, the variations experienced should all be controlled to a statistically insignificant level except for tool wear. That means you GR&R is less than 10% of your control limits (yes, control limits!), etc. If this is the case, then you are truly doing "precision machining" and your distribution is a uniform or rectangular distribution. To determine the long term sampling plan, you should perform a capability study of approximately 100 pcs in a row, in order!. If you are doing an OD, the OD should increase over time as the tool wears. You should set up control limits of 75% of the specification. As the dimension reaches the control limit, adjust down to the lower control limit and allow to resume increasing to the upper control limit. With a uniform distribution, the "mean" has no practical value for evaluating the process. To determine the sampling frequency, you need to divide the time or number of parts required to go from the lower control limit to the upper control limit, and divide by 5. As far as sampling number, you only need to sample 1 part per increment. Why? Because in precision machining there is so little difference between consecutive parts that it is a waste of time. You should spend more time measuring that one part, getting the highest and lowest diameter and plotting a X hi/lo-R chart. Then, you are actually controlling the GD&T parameters of a circular characteristics. Xbar-R chart is the worst possible chart for this distribution - it simply encourages overcontrol.

Also for precision machining, (uniform distribution) the capability is calculated as (USL-LSL)/(UCL-LCL) Note that it is a constant, because (UCL-LCL)=.75(USL-LSL) from using control limits set at 75% of the specification.

For more details on this see thread
http://elsmar.com/Forums/showthread.php?t=20935

HOWEVER - for a "traditional lathe" (if it is one that the operator moves the tool by hand), it may not fall into the category of "precision machining", and could very well be a normal distribution. When operators become the key variation of the process, it tends to have a more "natural variation" and fall within normal distribution, especially if they are doing the "backyard garage style control" of "running to the mean" to control their dimensions. :cool:
 
Last edited:

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
Hi All,

I'm a bit confused....
One of our customers are asking us to do a Machine Capability study !!!
And I asked if they refer to Cpk capability study (on product) and said NO.

Can somebody explain the differences between Machine capability and Product capability studies and how to perform (machine one) ?

Thanks a lot...:notme:

M.

My understanding of this issue (and this is for discussion, not a final answer!;)), is the difference between machine capability and product capability is the key term of "specification". Product capability is the product specification divided by the process performance. Machine capability should therefore be the machine specification (as provided by the manufacturer) versus the process performance. The hard part is figuring out the machine specification, especially on a 50 year old ACME Screw machine. And, what about in-house improvements to the machine? Do they count? Good question for your SQE.... :cool:

Your customer is looking for evidence that you are using the right machine for the job. Can you prove it? Do you have this information PRIOR to picking that machine for the job during the quoting processs? They want you to ponder this because they have been burned time and time again by suppliers that quote on a part, then take 3 months trying to get an incapable machine (because that's all they have or can afford) to make the part, only to fail miserably and put projects months behind schedule. Ouch - customers hate that.
 
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